Impact of crustal deformation detection by the DSI (difference of split-band interferograms) method with PALSAR-2 data: A case study on the 2016 Kumamoto Earthquake
Abstract Interferometric Synthetic Aperture Radar (InSAR) is a useful tool for detecting surface deformations at high spatial resolutions. When InSAR is applied to huge surface deformations, clear fringes with complicated phase gaps often appear in the interferograms. Although the surface deformations in such areas are important for understanding their mechanisms and for investigating disasters, it is difficult to convert the data on such fringes to surface deformation information because of difficulties associated with phase unwrapping. To resolve these difficulties, we created multiple SAR pairs with different frequencies using a band-pass filter and derived the difference of interferograms which are generated from these SAR pairs. Generally, its result corresponds to the result of SAR observations made with long-wavelength radar. Therefore, a phase wrap was less likely to occur, and phase unwrapping was easy to accomplish. We applied this method to the PALSAR-2 data pairs for the 2016 Kumamoto Earthquake and succeeded in identifying huge crustal deformations with complicated phase gaps in the vicinity of surface ruptures. Comparing these results with the crustal deformations observed from GNSS measurements, the root-mean-squares of the differences were found to be approximately 4 cm. Although this accuracy was poorer than that of conventional InSAR, it was nearly equivalent to that of the offsettracking method. Furthermore, its spatial resolution was significantly better than that of the offset-tracking method. However, the disadvantage of this method is that its detection accuracy is significantly degraded in zones with low coherence, due to noise amplification. The standard deviation of the noise component was approximately 2 cm for pixels with coherences above 0.7. However, for pixels with a coherence lower than 0.2, the standard deviation was greater than 10 cm, and the noise component occasionally exceeded 1 m. Despite the disadvantages of this method, it is effective for the detection of huge crustal deformations with high spatial resolution in areas where phase unwrapping methods for conventional InSAR are inappropriate.
